str_replace_editor doesn't preserve the encoding properly for non-ASCII characters.

### Describe the bug

Run into a issue where "ä" was not encoded properly after edit and shows up as triangle with question mark . Copilot itself reported this to be caused by " str_replace_editor doesn't preserve
the encoding properly for non-ASCII characters."

### Affected version

_No response_

### Steps to reproduce the behavior

1. Ask Copilot to modify C# files with comments if Finnish for example "// Täysi"
2. Copilot does changes which include the comment line
3. "ä" character is corrupted

### Expected behavior

No character corruption

### Additional context

```pwsh
copilot ---version
0.0.341
Commit: 5725358
```